1. Field of the Invention
The present invention relates to techniques for locating obscured objects, and more specifically, it relates to techniques for imaging objects located within a scattering medium.
2. Description of Related Art
Medical personnel often need to insert a needle or other similar devise in a near surface vein of a patient; however, it is often difficult to see or locate the vein and/or successfully insert the needle into the vein. This problem arises mainly due to the difficulty to visualize the vascular structures via observation with the naked eye because the ability of visible photons to penetrate the tissue is very limited. The use of infrared light to improve the visibility of the near surface vascular system is widely known. Reduced absorption of blood and myoglobin of the NIR spectral region allows a larger photon penetration depth of the NIR photons in the tissue. Photons in this spectral region can reach and interact with the subsurface veins such that images are formable from the detected NIR photons. Blood, however, remains the main absorbing chromophore, thus causing the veins to appear as darker features independently of the illumination wavelength.
The problem of visualizing the subsurface vascular system even with NIR light arises from that fact that still only a small portion of the light injected though the surface is able to reach the vein before being backscattered to reach the imaging devise. This process is depicted in FIG. 1. Specifically, upon the illumination with NIR light (from source 10) of the tissue (arm 12), a portion of the light 14 will be reflected at the interface between tissue and air due to the change in the index of refraction. The resulting image component (specular reflection image or SR-image) has no information on the spatial characteristics of the vein since it never interacted with the vein 16 of arm 12. The same is true for a second image component 18 that is composed of photons that only reached tissue depths shorter than the depth that the vein is located. The resulting image component (near subsurface or NS-image) is stronger than the third image component 20 (deep subsurface or DS-image) that is composed by photons that reached an adequate depth to interact with the vein. The DS-image bears information about the vein presence and geometrical characteristics and can be recorded by the imaging device 22. The relative intensity (strength) of the NS and DS image components depends on two parameters: i) the illumination wavelength (arising from the dependence of the reduced scattering coefficient based on the wavelength) and ii) the depth of the vein (since the number of photons that can a reach certain depth declines nearly exponentially with depth). As a result, if the vein is located deep enough below the surface, only a very small portion of the overall number of detected photons by the imaging device have interacted with the vein, which results in inadequate image contrast to visualize the vein. Therefore, in order to improve the ability to image the vein, the relative contribution of the DS image component must be increased. In previous work by Demos et al., methods were proposed to reduce or eliminate the SR and the NS image components. Specifically, since the SR image component is due to a single reflection event per detected photon, these photons maintain their polarization state. Therefore, using polarized illumination and detecting the orthogonal image components, the SR image component is at least partially eliminated. In order to remove the NS image component, the Spectral and Polarization Difference Imaging (SPDI) technique was proposed where the orthogonal polarization image components at two different wavelengths is used and subtracted after they are normalized so that the intensity of their corresponding NS image components is approximately equal. This subtraction provides the difference image of the remaining DS image components that are different in intensity due to the use of different wavelengths. It is desirable to overcome the degree of difficulty imposed by these prior art methods.